Question,
besides retaining cooliant, what other purpose does the cap serve, and if the inside rubber seal breaks on a cap will that alter the cars performance?..I have a 1988 none turbo RX7, and I recently noticed that the cap that resides at the top of the engine...looked to be cracked...worn..broken.

 

The rad cap is what decides when to flow coolant to and from the overflow tank. It uses coolant pressure versus a spring in the cap to open and close a valve. It is very important to the health of the car, so if it is damaged, replace it.

 

When coolant heats up it expands, increasing the pressure in the closed cooling system. The radiator cap allows the pressure to build up to its rated pressure (13psi for our cars) and then bleeds coolant into the overflow tank to maintain that pressure. When the coolant cools and contracts, the radiator cap allows the coolant in the overflow tank to be sucked back into the radiator.
Like Aaron said, it’s very important that this is working properly. Always replace it if you suspect its not working properly, like if the coolant isn’t returning to the radiator.

 

i got a question.about the s4 cap on the thermostat housing . i have an 86 na s4. i have the one with no filler cap on the rad,just the one on the thermostat housing,should this cap be a pressure release cap or just a plain cap? i read while searching that its a plain cap,and only the radiator filler cap on s5's needs the pressure release cap,could someone clear this up,what cap should i be running? --there is a line to the resevior under my cap,so that would tell me a i need a pressure release one,but im never right,so HELP!?!?

 

har har just found out,nevermind